Copolymer of ethylene and a 1,3-diene

ABSTRACT

A copolymer of ethylene and of a 1,3-diene of formula CH2═CR—CH═CH2 is provided. The ethylene units represent between 50 mol % and 95 mol % of the ethylene units and of the units of the 1,3-diene, and the units of the 1,3-diene of 1,2 and 3,4 configuration represent more than 50 mol % of the units of the 1,3-diene. The symbol R represents a hydrocarbon chain having from 3 to 20 carbon atoms. Such a copolymer exhibits an improved compromise between the degree of crystallinity and the stiffness and makes it possible to widen the field of application of ethylene-rich diene copolymers in rubber compositions.

This application is a 371 national phase entry of PCT/FR2019/050598filed on 18 Mar. 2019, which claims benefit of French Patent ApplicationNo. 1852305, filed 19 Mar. 2018, the entire contents of which areincorporated herein by reference for all purposes.

BACKGROUND 1. Technical Field

The field of the invention is that of copolymers of conjugated diene andof ethylene, rich in ethylene unit and usable as elastomers in a rubbercomposition for tires.

2. Related Art

The most widely used diene elastomers in the manufacture of tires arepolybutadienes, polyisoprenes, in particular natural rubber, andcopolymers of 1,3-butadiene and of styrene. The point common to theseelastomers is the high molar proportion of diene units in the elastomer,generally much greater than 50%, which can render them sensitive tooxidation, in particular under the action of ozone.

The Applicant Company has described elastomers which, on the contrary,are relatively poor in diene units, in particular for the purpose ofreducing their sensitivity to oxidation phenomena. These elastomers are,for example, described in the document WO 2007054223. These arecopolymers of 1,3-butadiene and of ethylene containing more than 50 mol% of ethylene unit. These elastomers are described as ethylene-richdiene elastomers.

Ethylene-rich copolymers of 1,3-butadiene and of ethylene arecrystalline and experience an increase in their their crystallinity withthe content of ethylene. The presence of crystalline parts in thecopolymer can be problematic when the copolymer is used in a rubbercomposition. As the melting of the crystalline parts of the copolymerresults in a fall in its stiffness, a rubber composition containing sucha copolymer and used in a tire will also experience a decrease in itsstiffness when it is brought to temperatures equalling or exceeding themelting point of the crystalline parts, which may be the case duringrepeated phases of braking and of acceleration of the tire. Thisdependency of the stiffness as a function of the temperature can thusresult in uncontrolled fluctuations in the performance qualities of thetire. It is advantageous to have available diene polymers rich inethylene units, the crystallinity of which is reduced, indeed eveneliminated.

In the document WO 2007054224, the Applicant Company has describedethylene-rich diene copolymers which exhibit a reduced crystallinity.These copolymers are copolymers of 1,3-butadiene and of ethylene whichadditionally contain saturated 6-membered cyclic hydrocarbon motifs.Nevertheless, these copolymers introduced into a rubber composition canconfer an excessively high stiffness on the rubber composition. The highstiffness of the rubber composition is attributed to an equally highstiffness of the elastomer. A high stiffness of a rubber composition canbe problematic as it can itself also render the rubber compositionunsuitable for certain applications.

In order to produce these copolymers of ethylene and of 1,3-butadienerich in ethylene and comprising saturated 6-membered cyclic hydrocarbonmotifs, the Applicant Company has developed a catalytic system based ona metallocene and on an organomagnesium compound, as is described, forexample, in the document WO 2007054224, the metallocene being offollowing formula:P(Cp¹)(Cp²)Nd(BH₄)_((1+y))-L_(y)-N_(x)

-   -   Cp¹ and Cp², which are identical or different, being selected        from the group consisting of substituted fluorenyl groups and        the unsubstituted fluorenyl group of formula C₁₃H₈, P being a        group bridging the two Cp¹ and Cp² groups and representing a        ZR³R⁴ group, Z representing a silicon or carbon atom, R³ and R⁴,        which are identical or different, each representing an alkyl        group comprising from 1 to 20 carbon atoms, preferably a methyl,        y, an integer, being equal to or greater than 0, x, an integer        or non-integer, being equal to or greater than 0, L representing        an alkali metal selected from the group consisting of lithium,        sodium and potassium, and N representing a molecule of an ether,        preferably diethyl ether or tetrahydrofuran.

SUMMARY

Pursuing its aim of synthesizing ethylene-rich diene elastomers, theApplicant Company has discovered a new polymer which makes it possibleto solve the problems mentioned.

Thus, a first subject-matter of the invention is a copolymer, preferablyan elastomer, of ethylene and of 1,3-diene of formula (I) whichcomprises ethylene units and units of the 1,3-diene, the ethylene unitsrepresenting between 50 mol % and 95 mol % of the ethylene units and ofthe units of the 1,3-diene, and the units of the 1,3-diene of 1,2 and3,4 configuration representing more than 50 mol % of the units of the1,3-diene,CH₂═CR—CH═CH₂  (I)the symbol R representing a hydrocarbon chain having from 3 to 20 carbonatoms.

Another subject-matter of the invention is a process for the preparationof the copolymer in accordance with the invention.

The invention also relates to a rubber composition based at least on theelastomer in accordance with the invention and on a crosslinking system,as well as to a tire which comprises the rubber composition inaccordance with the invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

In the present description, any interval of values denoted by theexpression “between a and b” represents the range of values greater than“a” and lower than “b” (that is to say, limits a and b excluded),whereas any interval of values denoted by the expression “from a to b”means the range of values extending from “a” up to “b” (that is to say,including the strict limits a and b).

The expression “based on” used to define the constituents of a catalyticsystem or of a composition is understood to mean the mixture of theseconstituents, or the product of the reaction of a portion or of all ofthese constituents with one another.

Unless otherwise indicated, the contents of the units resulting from theinsertion of a monomer into a copolymer are expressed as molarpercentage with respect to all of the monomer units of the copolymer.

The compounds mentioned in the description can be of fossil origin or bebiosourced. In the latter case, they can result, partially orcompletely, from biomass or be obtained from renewable startingmaterials resulting from biomass. The monomers are concerned inparticular.

As the 1,3-diene of formula (I) as defined above and of use for therequirements of the invention is a substituted 1,3-diene, the 1,3-dienecan give rise to units of 1,2 configuration represented by the formula(1), of 3,4 configuration represented by the formula (2) and of 1,4configuration, the trans form of which is represented below by theformula (3).

As is also well known, the ethylene unit is a unit of —(CH₂—CH₂)— motif.

The copolymer in accordance with the invention is a copolymer ofethylene and of the 1,3-diene, which implies that the monomer units ofthe copolymer are units resulting from the polymerization of ethyleneand of the 1,3-diene. The copolymer thus comprises ethylene units andunits of the 1,3-diene. According to any one of the embodiments of theinvention, the 1,3-diene of use for the requirements of the invention isjust one compound, that is to say just one 1,3-diene of formula (I), oris a mixture of 1,3-dienes of formula (I), the 1,3-dienes of the mixturediffering from one another by the group represented by the symbol R.

The essential characteristic of the copolymer of ethylene and of the1,3-diene in accordance with the invention is to comprise between 50 mol% and 95 mol % of ethylene unit. In other words, the ethylene unitsrepresent between 50 mol % and 95 mol % of the ethylene units and of the1,3-diene units. Another essential characteristic is also to compriseunits of the 1,3-diene which are, to more than 50 mol %, units of the1,3-diene of 1,2 and 3,4 configuration. In other words, the units of the1,3-diene, whether they are of 1,2 or 3,4 configuration, represent morethan 50 mol % of the units of the 1,3-diene. The remainder to 100 mol %of the units of the 1,3-diene in the copolymer is completely orpartially formed of units of the 1,3-diene of 1,4 configuration.According to any one of the embodiments of the invention, preferentiallymore than half of the units of the 1,3-diene of 1,4 configuration are oftrans-1,4 configuration, more preferentially all the units of the1,3-diene of 1,4 configuration are of trans-1,4 configuration.

In the formula (I) of the 1,3-diene of use for the requirements of theinvention, the hydrocarbon chain represented by the symbol R can be alinear or branched chain, in which case the symbol R represents a linearor branched chain. Preferably, the hydrocarbon chain is acyclic, inwhich case the symbol R represents an acyclic chain. In the formula (I),the hydrocarbon chain represented by the symbol R can be saturated orunsaturated, in which case the symbol R represents a saturated orunsaturated chain. Preferably, the symbol R represents a hydrocarbonchain having from 6 to 16 carbon atoms.

According to a preferential embodiment of the invention, in thecopolymer in accordance with the invention, the ethylene units representat least 60 mol % of the ethylene units and of the units of the1,3-diene. More preferentially, the ethylene units represent from 60 mol% to 90 mol % of the ethylene units and of the units of the 1,3-diene.

According to a more preferential embodiment of the invention, in thecopolymer in accordance with the invention, the ethylene units representat least 70 mol % of the ethylene units and of the units of the1,3-diene. More preferentially, the ethylene units represent from 70 mol% to 90 mol % of the ethylene units and of the units of the 1,3-diene.

Preferably, the copolymer in accordance with the invention has a glasstransition temperature of less than −35° C., in particular of between−90° C. and −35° C.

More preferentially, the copolymer in accordance with the invention isan elastomer.

The copolymer in accordance with the invention can be prepared by aprocess which comprises the copolymerization of ethylene and of the1,3-diene in the presence of a catalytic system based at least on ametallocene of formula (II) and on an organomagnesium compound offormula (III)P(Cp¹Cp²)Nd(BH₄)_((1-y))-L_(y)-N_(x)  (II)MgR¹R²  (III)

-   -   C^(p1) and Cp², which are identical or different, being selected        from the group consisting of substituted fluorenyl groups and        the unsubstituted fluorenyl group of formula C₁₃H₈, P being a        group bridging the two Cp¹ and Cp² groups and representing a        ZR³R⁴ group, Z representing a silicon or carbon atom, R³ and R⁴,        which are identical or different, each representing an alkyl        group comprising from 1 to 20 carbon atoms, preferably a methyl,    -   y, which is an integer, being equal to or greater than 0,    -   x, which is or is not an integer, being equal to or greater than        0,    -   L representing an alkali metal selected from the group        consisting of lithium, sodium and potassium,    -   N representing a molecule of an ether, preferably diethyl ether        or tetrahydrofuran,    -   R¹ and R², which are identical or different, representing a        carbon group.

Mention may be made, as substituted fluorenyl groups, of thosesubstituted by alkyl radicals having from 1 to 6 carbon atoms or by arylradicals having from 6 to 12 carbon atoms. The choice of the radicals isalso guided by the accessibility to the corresponding molecules, whichare the substituted fluorenes, because the latter are commerciallyavailable or can be easily synthesized.

Mention may more particularly be made, as substituted fluorenyl groups,of the 2,7-di(tert-butyl)fluorenyl and 3,6-di(tert-butyl)fluorenylgroups. The 2, 3, 6 and 7 positions respectively denote the positions ofthe carbon atoms of the rings as represented in the scheme below, the 9position corresponding to the carbon atom to which the bridge P isattached.

The catalytic system can be prepared conventionally by a processanalogous to that described in Patent Application WO 2007054224. Forexample, the organomagnesium compound and the metallocene can be reactedin a hydrocarbon solvent typically at a temperature ranging from 20 to80° C. for a period of time of between 5 and 60 minutes. The catalyticsystem is generally prepared in an aliphatic hydrocarbon solvent, suchas methylcyclohexane, or an aromatic hydrocarbon solvent, such astoluene. Generally, after its synthesis, the catalytic system is used inthis form in the process for the synthesis of the copolymer inaccordance with the invention.

The metallocene used for preparing the catalytic system can be in theform of a crystalline or non-crystalline powder, or else in the form ofsingle crystals. The metallocene can be provided in a monomer or dimerform, these forms depending on the method of preparation of themetallocene, as for example is described in Patent Application WO2007054224. The metallocene can be prepared conventionally by a processanalogous to that described in Patent Application WO 2007054224, inparticular by reaction, under inert and anhydrous conditions, of thesalt of an alkali metal of the ligand with a rare earth metalborohydride in a suitable solvent, such as an ether, for example diethylether or tetrahydrofuran, or any other solvent known to a person skilledin the art. After reaction, the metallocene is separated from thereaction by-products by the techniques known to a person skilled in theart, such as filtration or precipitation from a second solvent. In theend, the metallocene is dried and isolated in the solid form.

Like any synthesis carried out in the presence of an organometalliccompound, the synthesis of the metallocene and that of the catalyticsystem take place under anhydrous conditions under an inert atmosphere.Typically, the reactions are carried out starting from anhydroussolvents and compounds under anhydrous nitrogen or argon.

Preferably, the metallocene is of formula (IIa), (IIb), (IIc), (IId) or(IIe), in which the symbol Flu represents the fluorenyl group of formulaC₁₃H₈.[{Me₂SiFlu₂Nd(μ-BH₄)₂Li(THF)}₂]  (IIa)[Me₂SiFlu₂Nd(μ-BH₄)₂Li(THF)]  (IIb)[Me₂SiFlu₂Nd(μ-BH₄)(THF)]  (11c)[{Me₂SiFlu₂Nd(μ-BH₄)(THF)}₂]  (11d)[Me₂SiFlu₂Nd(μ-BH₄)]  (11e)

The organomagnesium compound of use for the requirements of theinvention is of formula MgR¹R² in which R¹ and R², which are identicalor different, represent a carbon group. Carbon group is understood tomean a group which contains one or more carbon atoms. Preferably, R¹ andR² contain from 2 to 10 carbon atoms. More preferentially, R¹ and R²each represent an alkyl. The organomagnesium compound is advantageouslya dialkylmagnesium compound, better still butylethylmagnesium orbutyloctylmagnesium, even better still butyloctylmagnesium.

According to any one of the embodiments of the invention, the molarratio of the organomagnesium compound to the metal Nd constituting themetallocene is preferably within a range extending from 1 to 100, morepreferably is greater than or equal to 1 and less than 10. The range ofvalues extending from 1 to less than 10 is in particular more favourablefor obtaining copolymers of high molar masses.

A person skilled in the art also adapts the polymerization conditionsand the concentrations of each of the reactants (constituents of thecatalytic system, monomers) according to the equipment (devices,reactors) used to carry out the polymerization and the various chemicalreactions. As is known to a person skilled in the art, thecopolymerization and the handling of the monomers, of the catalyticsystem and of the polymerization solvent(s) take place under anhydrousconditions and under an inert atmosphere. The polymerization solventsare typically aliphatic or aromatic hydrocarbon solvents.

The polymerization is preferably carried out in solution, continuouslyor batchwise. The polymerization solvent can be an aromatic or aliphatichydrocarbon solvent. Mention may be made, as example of polymerizationsolvent, of toluene and methylcyclohexane. The monomers can beintroduced into the reactor containing the polymerization solvent andthe catalytic system or, conversely, the catalytic system can beintroduced into the reactor containing the polymerization solvent andthe monomers. The copolymerization is typically carried out underanhydrous conditions and in the absence of oxygen, in the optionalpresence of an inert gas. The polymerization temperature generallyvaries within a range extending from 30 to 150° C., preferentially from30 to 120° C. Preferably, the copolymerization is carried out atconstant ethylene pressure.

The polymerization can be halted by cooling the polymerization medium.The polymer can be recovered according to conventional techniques knownto a person skilled in the art, such as, for example, by precipitation,by evaporation of the solvent under reduced pressure or by steamstripping.

According to any one of the embodiments of the invention, theincorporation of the 1,3-diene and of the ethylene into the growingpolymer chain is preferentially random. The copolymer in accordance withthe invention is advantageously a random copolymer.

The copolymer in accordance with the invention, in particular when it isan elastomer, can be used in a rubber composition.

The rubber composition, which is another subject-matter of theinvention, has the characteristic of comprising the elastomer inaccordance with the invention and a crosslinking system.

The crosslinking system can be based on sulfur, on sulfur donors, onperoxides, on bismaleimides or on their mixtures. The crosslinkingsystem is preferentially a vulcanization system, that is to say a systembased on sulfur (or on a sulfur donor) and on a primary vulcanizationaccelerator. Additional to this base vulcanization system are optionallyvarious known secondary vulcanization accelerators or vulcanizationactivators, such as zinc oxide, stearic acid or equivalent compounds, orguanidine derivatives (in particular diphenylguanidine), or also knownvulcanization retarders.

According to a preferential embodiment of the invention, the rubbercomposition comprises a reinforcing filler. The rubber composition cancomprise any type of “reinforcing” filler known for its abilities toreinforce a rubber composition which can be used for the manufacture oftires, for example an organic filler, such as carbon black, areinforcing inorganic filler, such as silica, with which is combined, ina known way, a coupling agent, or also a mixture of these two types offiller. Such a reinforcing filler typically consists of nanoparticles,the (weight-) average size of which is less than a micrometre, generallyless than 500 nm, most often between 20 and 200 nm, in particular andmore preferentially between 20 and 150 nm. The content of reinforcingfiller is adjusted by a person skilled in the art according to the useof the rubber composition.

The rubber composition can additionally contain other additives known tobe used in rubber compositions for tires, such as plasticizers,antiozonants or antioxidants.

The rubber composition in accordance with the invention is typicallymanufactured in appropriate mixers, using two successive phases ofpreparation well known to a person skilled in the art: a first phase ofthermomechanical working or kneading (“non-productive” phase) at hightemperature, up to a maximum temperature of between 130° C. and 200° C.,followed by a second phase of mechanical working (“productive” phase) upto a lower temperature, typically of less than 110° C., for examplebetween 40° C. and 100° C., during which finishing phase thecrosslinking system is incorporated.

The rubber composition in accordance with the invention, which can beeither in the raw state (before crosslinking or vulcanization) or in thecured state (after crosslinking or vulcanization), can be used in a tiresemi-finished article.

The tire, which is another subject-matter of the invention, comprisesthe rubber composition in accordance with the invention defined underany one of the embodiments of the invention.

A better understanding of the abovementioned characteristics of thepresent invention, and also others, will be obtained on reading thefollowing description of several implementation examples of theinvention, given by way of illustration and without limitation.

IMPLEMENTATIONAL EXAMPLES OF THE INVENTION

1) Synthesis of the Polymers:

In the synthesis of copolymers in accordance with the invention, the1,3-diene used (myrcene) is a 1,3-diene of formula (I) in which R is ahydrocarbon group having 6 carbon atoms of formula CH₂—CH₂—CH═CMe₂.

All the reactants are obtained commercially, except the metallocenes[{Me₂SiFlu₂Nd(μ-BH₄)₂Li(THF)}] and [Me₂SiCpFluNd(μ-BH₄)₂Li(THF)], whichare prepared according to the procedures described in PatentApplications WO 2007054224 and WO 2007054223.

The butyloctylmagnesium BOMAG (20% in heptane, C=0.88 mol·l⁻¹)originates from Chemtura and is stored in a Schlenk tube under an inertatmosphere. The ethylene, of N35 grade, originates from Air Liquide andis used without prepurification. The myrcene (purity ≥95%) is obtainedfrom Sigma-Aldrich.

1.1—Control Synthesis: Example 1

The polymer is synthesized according to the following procedure:

The cocatalyst, the butyloctylmagnesium (BOMAG) and then the metallocene[Me₂SiCpFluNd(μ-BH₄)₂Li(THF)] are added to a 500-ml glass reactorcontaining 300 ml of toluene. The alkylation time is 10 minutes and thereaction temperature is 20° C. The respective amounts of theconstituents of the catalytic system appear in Table 2. Subsequently,the monomers are added according to the respective proportions shown inTable 2, the ethylene (Eth) and the 1,3-butadiene (Bde) being in theform of a gaseous mixture. The polymerization is carried out at 80° C.and at a constant ethylene pressure of 4 bars.

The polymerization reaction is halted by cooling, degassing of thereactor and addition of 10 ml of ethanol. An antioxidant is added to thepolymer solution. The copolymer is recovered by drying in an oven undervacuum to constant weight. The weight weighed makes it possible todetermine the mean catalytic activity of the catalytic system, expressedin kilograms of polymer synthesized per mole of neodymium metal and perhour (kg/mol·h).

1.2—Example not in Accordance with the Invention: Example 2

The polymer is synthesized according to the following procedure:

The cocatalyst, the butyloctylmagnesium (BOMAG) and then the metallocene[Me₂Si(Flu)₂Nd(μ-BH₄)₂Li(THF)] are added to a 500-ml glass reactorcontaining 300 ml of methylcyclohexane. The alkylation time is 10minutes and the reaction temperature is 20° C. The respective amounts ofthe constituents of the catalytic system appear in Table 2.Subsequently, the monomers are added according to the respectiveproportions shown in Table 2, the ethylene (Eth) and the 1,3-butadiene(Bde) being in the form of a gaseous mixture. The polymerization iscarried out at 80° C. and at a constant ethylene pressure of 4 bars. Thepolymerization reaction is halted by cooling, degassing of the reactorand addition of 10 ml of ethanol. An antioxidant is added to the polymersolution. The copolymer is recovered by drying in an oven under vacuumto constant weight. The weight weighed makes it possible to determinethe mean catalytic activity of the catalytic system, expressed inkilograms of polymer synthesized per mole of neodymium metal and perhour (kg/mol·h).

1.3—Examples in Accordance with the Invention: Examples 3 to 5

The polymers are synthesized according to the following procedure:

The cocatalyst, the butyloctylmagnesium (BOMAG) and then the metallocene[Me₂Si(Flu)₂Nd(μ-BH₄)₂Li(THF)] are added to a 500-ml glass reactorcontaining 300 ml of methylcyclohexane. The alkylation time is 10minutes and the reaction temperature is 20° C. The respective amounts ofthe constituents of the catalytic system appear in Table 2.Subsequently, the myrcene is added to the reactor before the injectionof the gaseous ethylene. The polymerization is carried out at 80° C. andat a constant ethylene pressure of 4 bars.

The polymerization reaction is halted by cooling, degassing of thereactor and addition of 10 ml of ethanol. An antioxidant is added to thepolymer solution. The copolymer is recovered by drying in an oven undervacuum to constant weight. The weight weighed makes it possible todetermine the mean catalytic activity of the catalytic system, expressedin kilograms of polymer synthesized per mole of neodymium metal and perhour (kg/mol·h).

The characteristics of the polymers appear in Tables 3 and 4.

2) Determination of the Microstructure of the Polymers:

The spectral characterization and the measurements of the microstructureof copolymer of ethylene and of the 1,3-diene (myrcene) are carried outby nuclear magnetic resonance (NMR) spectroscopy.

-   -   Spectrometer: For these measurements, a Bruker Avance III HD 400        MHz spectrometer is used, equipped with a Bruker cryo-BBFO        z-grad 5 mm probe.    -   Experiments: The ¹H experiments are recorded using a        radiofrequency pulse with a tilt angle of 30°, the number of        repetitions is 128 with a recycle delay of 5 seconds. The HSQC        (Heteronuclear Single Quantum Coherence) and HMBC (Heteronuclear        Multiple-Bond Correlation) ¹H-¹³C NMR correlation experiments        are recorded with a number of repetitions of 128 and a number of        increments of 128. The experiments are carried out at 25° C.    -   Preparation of the sample: 25 mg of sample are dissolved in 1 ml        of deuterated chloroform (CDCl₃).    -   Calibration of the sample: The axes of the ¹H and ¹³C chemical        shifts are calibrated with respect to the protonated impurity of        the solvent (CHCl₃) at δ_(1H)=7.2 ppm and δ_(13C)=77 ppm.    -   Spectral assignment: The signals of the insertion forms of the        1,3-diene A, B and C (Scheme 1) were observed on the different        spectra recorded. According to S. Georges et al. (S. Georges, M.        Bria, P. Zinck and M. Visseaux, Polymer, 55 (2014), 3869-3878),        the signal of the —CH═ group No. 8″ characteristic of the form C        exhibits ¹H and ¹³C chemical shifts identical to the —CH═ group        No. 3.

The chemical shifts of the signals characteristic of the motifs A, B andC are presented in Table 1. The motifs A, B and C correspondrespectively to the units of 3,4 configuration, of 1,2 configuration andof trans-1,4 configuration.

TABLE 1 Assignment of the ¹H and ¹³C signals of Ethylene/Myrcenecopolymers δ_(1H) (ppm) δ_(13C) (ppm) Group 5.54 146.4  8′ 5.07 124.63 + 8″ 4.97-4.79 112.0  9′ 4.67 108.5 7 2.06  26.5 4 2.0-1.79  31.8 5 +5′ + 5″  44.5 8 1.59 25.9 and 17.0 1 1.2  36.8-24.0 CH₂ ethylene

The quantifications were carried out from the integration of the 1D ¹HNMR spectra using the Topspin software.

The integrated signals for the quantification of the different motifsare:

-   -   Ethylene: signal at 1.2 ppm corresponding to 4 protons    -   Total myrcene: signal No. 1 (1.59 ppm) corresponding to 6        protons    -   Form A: signal No. 7 (4.67 ppm) corresponding to 2 protons    -   Form B: signal No. 8′ (5.54 ppm) corresponding to 1 proton

The quantification of the microstructure is carried out in molarpercentage (molar %) as follows: Molar % of a motif=¹H integral of amotif*100/Σ(¹H integrals of each motif).

3) Determination of the Stiffness of the Polymers (in the Raw State):

The measurements are carried out on an Anton Paar model MCR301 rheometerin shear mode with cylindrical test specimens of controlled geometry(thickness of between 1.5 mm and 3 mm and diameter of between 22 mm and28 mm). The sample is subjected to a sinusoidal shear stress, at a fixedtemperature (corresponding to the end of the passage of the glasstransition of the elastomer over a temperature sweep at 10 Hz), and overa frequency range extending from 0.01 Hz to 100 Hz. The stiffness valueselected as being the stiffness of the rubbery plateau of the sample isthe value of the shear modulus G′ for the frequency at which the lossmodulus G″ reaches its minimum, in accordance with the method describedby C. Liu, J. He, E. van Ruymbeke, R. Keunings and C. Bailly, Evaluationof different methods for the determination of the plateau modulus andthe entanglement molecular weight, Polymer, 47 (2006), 4461-4479.

4) Determination of the Glass Transition Temperature of the Polymers:

The glass transition temperature is measured by means of a differentialcalorimeter (differential scanning calorimeter) according to StandardASTM D3418 (1999).

5) Determination of the Degree of Crystallinity of the Polymers:

Standard ISO 11357-3:2011 is used to determine the temperature andenthalpy of fusion and of crystallization of the polymers used bydifferential scanning calorimetry (DSC). The reference enthalpy ofpolyethylene is 277.1 J/g (according to Polymer Handbook, 4th Edition,J. Brandrup, E. H. Immergut and E. A. Grulke, 1999).

6 Results:

In Example 1 (control), the diene copolymer rich in ethylene andsynthesized by polymerization of ethylene and of the 1,3-butadiene inthe presence of the metallocene [Me₂SiCpFluNd(μ-BH₄)₂Li(THF)] exhibits ahigh crystallinity (31%), which can render it unsuitable for some uses.

In Example 2 (not in accordance), the diene copolymer rich in ethylenesynthesized in the presence of the metallocene[Me₂Si(Flu)₂Nd(μ-BH₄)₂Li(THF)] exhibits cyclic motifs. Although itcontains an ethylene content comparable to that of the control, it isnot crystalline. Nevertheless, it has a relatively high stiffness, whichcan render it unsuitable for some uses.

In Examples 3 to 5 (in accordance), the diene copolymers rich inethylene are copolymers of ethylene and of myrcene. In Example 3, thecopolymer has an ethylene content comparable to that of the copolymersof Examples 1 and 2, but without exhibiting their disadvantages. This isbecause it has the advantage both of not being crystalline and of havinga significantly lower stiffness than the copolymer of Example 2.

In Example 4, the copolymer is much richer in ethylene (85%) than thecontrol copolymer of Example 1 (74%) and yet it is much less crystalline(17%) than the control copolymer (31%). In Example 5, the copolymer hasa higher myrcene content than the copolymers of Examples 3 and 4. It isnot crystalline and also exhibits a lower stiffness. Examples 3 to 5illustrate that a variation in the myrcene content in the copolymermakes it possible to improve the degree of crystallinity/stiffnesscompromise of ethylene-rich diene polymers, in comparison with thecopolymers of ethylene and of 1,3-butadiene.

To sum up, the replacement of 1,3-butadiene by a 1,3-diene of formulaCH₂═CR—CH═CH₂, R representing a hydrocarbon chain having from 3 to 20carbon atoms, such as myrcene, makes it possible to synthesizeethylene-rich diene polymers with an improved compromise between thedegree of crystallinity and the stiffness and to widen the field ofapplication of ethylene-rich diene copolymers in rubber compositions.

TABLE 2 Metallocene Alkylating agent Amount of Gas mixture Weight ofconcentration concentration myrcene composition Activity polymer Example(mmol/l) (mmol/l) (ml) (mol % Eth/Bde) (kg/mol · h) (g) Example 1 0.320.97 0 80/20 92 12.4 Example 2 0.16 0.78 0  70-30 134 12.9 Example 30.16 0.78 18 100-0 400 17.2 Example 4 0.16 0.78 10.3 100-0 300 17.1Example 5 0.16 0.32 50 100-0 250 18.8

TABLE 3 1,4 Myrcene 1,2 Myrcene 3,4 Myrcene Ethylene Butadiene1,2-Cyclohexanediyl Myrcene (mol %/mol % (mol %/mol % (mol %/mol %Examples (mol %) (mol %) (mol %) (mol %) myrcene) myrcene) myrcene)Example 1 73.5 26.5 Example 2 71 16 13 Example 3 74 26 31 4 65 Example 485 15 33 7 60 Example 5 64 36 31 3 66

TABLE 4 Stiffness in the raw Tg Crystallinity state Examples (° C.) (%)(MPa) Example 1 −54 31 — Example 2 −35 0 1 Example 3 −64 0 0.5 Example 4−62 17 — Example 5 −58 0 0.34

The invention claimed is:
 1. A process for the preparation of acopolymer of ethylene and of a 1,3-diene of formula (I) which comprisesethylene units and units of the 1,3-diene, the ethylene unitsrepresenting between 50 mol % and 95 mol % of the ethylene units and ofthe units of the 1,3-diene, and the units of the 1,3-diene of 1,2 and3,4 configuration representing more than 50 mol % of the units of the1,3-diene,CH₂=CR−CH=CH₂  (I) the symbol R representing a hydrocarbon chain havingfrom 3 to 20 carbon atoms, wherein the process comprises thepolymerization of ethylene and of the 1,3-diene in the presence of acatalytic system based at least on a metallocene of formula (II) and onan organomagnesium compound of formula (III)P(Cp¹Cp²)Nd(BH₄)_((1-y))-L_(y)-N_(x)  (II)MgR¹R²  (III) Cp¹ and Cp², which are identical or different, beingselected from the group consisting of substituted fluorenyl groups andthe unsubstituted fluorenyl group of formula C₁₃H₈, P being a groupbridging the two Cp¹ and Cp² groups and representing a ZR³R⁴ group, Zrepresenting a silicon or carbon atom, R³ and R⁴, which are identical ordifferent, each representing an alkyl group comprising from 1 to 20carbon atoms, y, which is an integer, being equal to or greater than 0,x, which is or is not an integer, being equal to or greater than 0, Lrepresenting an alkali metal selected from the group consisting oflithium, sodium and potassium, N representing a molecule of an ether, R¹and R², which are identical or different, representing a carbon group.2. The process according to claim 1, in which the metallocene is offormula (IIa), (IIb), (IIc), (IId) or (IIe)[{Me₂SiFlu₂Nd(μ-BH₄)₂Li(THF)}₂]  (IIa)[Me₂SiFlu₂Nd(μ-BH₄)₂Li(THF)]  (IIb)[Me₂SiFlu₂Nd(μ-BH₄)(THF)]  (IIc)[{Me₂SiFlu₂Nd(μ-BH₄)(THF)}₂]  (IId)[Me₂SiFlu₂Nd(μ-BH₄)]  (IIe) the symbol Flu representing the fluorenylgroup of formula C₁₃H₈.
 3. The process according to claim 1, in which R¹and R² contain from 2 to 10 carbon atoms.
 4. The process according toclaim 1, in which R¹ and R² each represent an alkyl.
 5. The processaccording to claim 1, in which the organomagnesium compound is adialkylmagnesium compound.
 6. The process according to claim 1, in whichthe ethylene units represent at least 60 mol % of the ethylene units andof the units of the 1,3-diene.
 7. The process according to claim 1, inwhich the ethylene units represent from 60 mol % to 90 mol % of theethylene units and of the units of the 1,3-diene.
 8. The processaccording to claim 1, in which the ethylene units represent from 70 mol% to 90 mol % of the ethylene units and of the units of the 1,3-diene.9. The process according to claim 1, in which the symbol R represents ahydrocarbon chain having from 6 to 16 carbon atoms.
 10. The processaccording to claim 1, in which the symbol R represents an acyclic chain.11. The process according to claim 1, in which the symbol R represents alinear or branched chain.
 12. The process according to claim 1, in whichthe symbol R represents a saturated or unsaturated chain.
 13. Theprocess according to claim 1, which copolymer has a glass transitiontemperature of less than −35° C.
 14. The process according to claim 1,which copolymer has a glass transition temperature of between −90° C.and —35° C.
 15. The process according to claim 1, which copolymer is arandom copolymer.
 16. The process according to claim 1, which copolymeris an elastomer.
 17. A rubber composition based at least on a copolymerdefined in claim 16 and on a crosslinking system.
 18. The rubbercomposition according to claim 17, which comprises a reinforcing filler.19. A tire which comprises a rubber composition defined in claim 17.